1. Field of the Invention
The present invention relates to an apparatus for measuring fundus fluorescence, more particularly to an apparatus for measuring natural fluorescence from the fundus of a subject eye.
2. Description of the Prior Art
Measuring fluorescence of eye tissues is used in ophthalmic diagnoses. Japanese Patent Laid Open Publication No. 1995-178055, for example, describes intravenously injecting a patient with a fluorescence agent, wherein a fundus camera is used to photograph fluorescence images in the fundus and measure the brightness of the fundus fluorescence to determine the state of the blood flow in the fundus. Japanese Patent Laid Open Publication No. 1995-8457 describes projecting exciting light onto the fundus and measuring the natural fluorescence (also called autofluorescence) given off by protein substances and oxidation degradation products, and using that as a basis for diagnosing the progression of diabetes and the like. Also, Japanese Patent Laid Open Publication No. 1997-28671 describes illuminating the cornea with exciting light, measuring the natural fluorescence coming from the cornea and using that as a non-invasive way of measuring blood glucose levels.
In age-related macular degeneration, a substance called lipofuscin accumulates in retinal macules. Lipofuscin is a type of fluorescence substance, so that it gives off natural fluorescence when illuminated with light of a specific wavelength. It is thought that measuring and grading the amount of natural fluorescence from the retina will enable early diagnosis of age-related macular degeneration.
However, the intensity of the natural fluorescence from the fundus is extremely low. In the method of Japanese Patent Laid Open Publication No. 1995-178055 in which a fundus camera is used without modification, a fluorescence agent is administered to the patient and the fluorescence from the fluorescence agent in the fundus is measured. In such a measurement it is possible to measure the fluorescence from the fluorescence agent, but impossible to measure the extremely weak natural fluorescence.
Also, to measure natural fluorescence from the fundus, exciting light is projected onto the fundus and the fluorescence has to be received via the cornea and crystalline lens, so that there is a considerable loss of intensity, and the amount of fluorescence received varies depending on the state of the eye concerned. Thus, the configuration of Japanese Patent Laid Open Publication No. 1995-8457 or No. 1997-28671 used to measure the natural fluorescence via the cornea and crystalline lens cannot be applied as-is to accurately measure the natural fluorescence from the desired portion of the fundus.
It is therefore an object of the present invention to provide an apparatus for measuring fundus fluorescence that can accurately measure natural fluorescence from a desired portion of an eye fundus.